feat: add email, scheduler, and Zulip integration services

- Add email service for sending meeting invites with ICS attachments
- Add scheduler for background calendar sync jobs
- Add Zulip service for meeting notifications
- Make ics_url optional for participants
- Add /api/schedule endpoint with 2-hour lead time validation
- Update frontend to support scheduling flow

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/package-lock.json

@@ -2859,7 +2859,6 @@
       "integrity": "sha512-bJFoMATwIGaxxx8VJPeM8TonI8t579oRvgAuT8zFugJsJZgzqv0Fu8Mhp68iecjzG7cnN3mO2dJQ5uUM2EFrgQ==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "undici-types": "~6.21.0"
       }
@@ -2877,7 +2876,6 @@
       "integrity": "sha512-/LDXMQh55EzZQ0uVAZmKKhfENivEvWz6E+EYzh+/MCjMhNsotd+ZHhBGIjFDTi6+fz0OhQQQLbTgdQIxxCsC0w==",
       "devOptional": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@types/prop-types": "*",
         "csstype": "^3.0.2"
@@ -2889,7 +2887,6 @@
       "integrity": "sha512-MEe3UeoENYVFXzoXEWsvcpg6ZvlrFNlOQ7EOsvhI3CfAXwzPfO8Qwuxd40nepsYKqyyVQnTdEfv68q91yLcKrQ==",
       "devOptional": true,
       "license": "MIT",
-      "peer": true,
       "peerDependencies": {
         "@types/react": "^18.0.0"
       }
@@ -2940,7 +2937,6 @@
       "integrity": "sha512-Zhy8HCvBUEfBECzIl1PKqF4p11+d0aUJS1GeUiuqK9WmOug8YCmC4h4bjyBvMyAMI9sbRczmrYL5lKg/YMbrcQ==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@typescript-eslint/scope-manager": "8.38.0",
         "@typescript-eslint/types": "8.38.0",
@@ -3173,7 +3169,6 @@
       "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "bin": {
         "acorn": "bin/acorn"
       },
@@ -3378,7 +3373,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "caniuse-lite": "^1.0.30001726",
         "electron-to-chromium": "^1.5.173",
@@ -3712,7 +3706,6 @@
       "resolved": "https://registry.npmjs.org/date-fns/-/date-fns-3.6.0.tgz",
       "integrity": "sha512-fRHTG8g/Gif+kSh50gaGEdToemgfj74aRX3swtiouboip5JDLAyDE9F11nHMIcvOaXeOC6D7SpNhi7uFyB7Uww==",
       "license": "MIT",
-      "peer": true,
       "funding": {
         "type": "github",
         "url": "https://github.com/sponsors/kossnocorp"
@@ -3794,8 +3787,7 @@
       "version": "8.6.0",
       "resolved": "https://registry.npmjs.org/embla-carousel/-/embla-carousel-8.6.0.tgz",
       "integrity": "sha512-SjWyZBHJPbqxHOzckOfo8lHisEaJWmwd23XppYFYVh10bU66/Pn5tkVkbkCMZVdbUE5eTCI2nD8OyIP4Z+uwkA==",
-      "license": "MIT",
-      "peer": true
+      "license": "MIT"
     },
     "node_modules/embla-carousel-react": {
       "version": "8.6.0",
@@ -3893,7 +3885,6 @@
       "integrity": "sha512-LSehfdpgMeWcTZkWZVIJl+tkZ2nuSkyyB9C27MZqFWXuph7DvaowgcTvKqxvpLW1JZIk8PN7hFY3Rj9LQ7m7lg==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@eslint-community/eslint-utils": "^4.2.0",
         "@eslint-community/regexpp": "^4.12.1",
@@ -5415,7 +5406,6 @@
         }
       ],
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "nanoid": "^3.3.11",
         "picocolors": "^1.1.1",
@@ -5602,7 +5592,6 @@
       "resolved": "https://registry.npmjs.org/react/-/react-18.3.1.tgz",
       "integrity": "sha512-wS+hAgJShR0KhEvPJArfuPVN1+Hz1t0Y6n5jLrGQbkb4urgPE/0Rve+1kMB1v/oWgHgm4WIcV+i7F2pTVj+2iQ==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "loose-envify": "^1.1.0"
       },
@@ -5629,7 +5618,6 @@
       "resolved": "https://registry.npmjs.org/react-dom/-/react-dom-18.3.1.tgz",
       "integrity": "sha512-5m4nQKp+rZRb09LNH59GM4BxTh9251/ylbKIbpe7TpGxfJ+9kv6BLkLBXIjjspbgbnIBNqlI23tRnTWT0snUIw==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "loose-envify": "^1.1.0",
         "scheduler": "^0.23.2"
@@ -5643,7 +5631,6 @@
       "resolved": "https://registry.npmjs.org/react-hook-form/-/react-hook-form-7.61.1.tgz",
       "integrity": "sha512-2vbXUFDYgqEgM2RcXcAT2PwDW/80QARi+PKmHy5q2KhuKvOlG8iIYgf7eIlIANR5trW9fJbP4r5aub3a4egsew==",
       "license": "MIT",
-      "peer": true,
       "engines": {
         "node": ">=18.0.0"
       },
@@ -6197,7 +6184,6 @@
       "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-3.4.17.tgz",
       "integrity": "sha512-w33E2aCvSDP0tW9RZuNXadXlkHXqFzSkQew/aIa2i/Sj8fThxwovwlXHSPXTbAHwEIhBFXAedUhP2tueAKP8Og==",
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "@alloc/quick-lru": "^5.2.0",
         "arg": "^5.0.2",
@@ -6322,7 +6308,6 @@
       "integrity": "sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==",
       "dev": true,
       "license": "Apache-2.0",
-      "peer": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
@@ -6502,7 +6487,6 @@
       "integrity": "sha512-qO3aKv3HoQC8QKiNSTuUM1l9o/XX3+c+VTgLHbJWHZGeTPVAg2XwazI9UWzoxjIJCGCV2zU60uqMzjeLZuULqA==",
       "dev": true,
       "license": "MIT",
-      "peer": true,
       "dependencies": {
         "esbuild": "^0.21.3",
         "postcss": "^8.4.43",